We consider congruences
between Eisenstein series and cusp forms — of weight k, level N and character χ of
conductor N — modulo large prime divisors of L(1 − k,χ−1). We show that such
primes occur in the order of a “global torsion” group attached to the cusp form f, and
(under a certain hypothesis) also in the denominator of the algebraic part of the
rightmost critical value Lf(k − 1). These occurrences are linked by the Bloch–Kato
conjecture.